The secret stairway into the dungeon is on the lower (non-fiction) aisle, controlled by a scroll on the shelf marked Mc-M. You need the library slip from the Mordred museum to access the stairway.
(Get the coin from the fountain, and pay your way into the museum. The slip is in a stack of books.)
Go to the library and click on Mc. It will reveal a secret passageway.
In the lower aisle of the castle library, in the Non-Fiction section. You cannot use the secret passageway until you have the Library Slip from books at the Mordred Museum.
The library slip is needed to get into the Dungeon through the castle library. It provides the location of the secret passageway. Get the gold coin from the fountain and pay your way into the Museum. The library slip is in a stack of books next to the museum operator.
go to the secret door
In the lower aisle of the castle library, in the Non-Fiction section. You cannot use the secret passageway until you have the Library Slip from books at the Mordred Museum. Get the gold coin from the fountain and find the slip in the museum.
There are two entrances to the dungeon : one is in the castle library, the other in the secret laboratory under the hay bales (this leads into the rear of the dungeon cell). (To enter them, see the related questions below.)
Push on the shelf marked "Non-Fiction McM" on the lower aisle of the library. This is written on the library slip found at the Mordred Museum, which you need to open the passageway. Get the shiny gold coin from the fountain and pay your admission, and find the library slip in the stack of books.
You will have to use the gold coin from the fountain to get the Library Slip from the Mordred Museum. Once you have it, you can click the McM shelf in the castle library and open the secret stairway to the dungeon.
It lets you access the castle dungeon. Use the gold coin to enter the Mordred museum and find the Library Slip. Go to the castle, where the library is to the right on the first floor. On the lower aisle, the Non-Fiction section, McM, there is a shelf that opens the secret dungeon passageway. (There are two things to do : get the moldy cheese and pull the lever.)
you go inside, get the moldy cheese and pull the lever down.
In the secret dungeon below the library is the captured robot attacker. But all you can do is pick up the moldy cheese and throw the switch. (This opens a rear gate on the cell.) Once you do this, you cannot go back in through the library passageway, and you need to find another way in. (see related question)
It is not a card, but a Library Slip, and it lets you access a secret passageway in the castle library. To get the slip, find the gold coin in the fountain next to the Museum. Use the coin to pay your admission. You will find the Library Slip in a stack of books. In the castle library (first floor right), go to the lower aisle of the library and press the shelf marked Mc-M. This will open a stairway into the dungeon.